A typical truck loading dock of a building includes an exterior doorway with an elevated platform for loading and unloading vehicles such as trucks and trailers. Many loading docks have a dock leveler to compensate for height differences between the loading dock platform and an adjacent bed of the truck or trailer. A typical dock leveler includes a deck, also known as a ramp or dockboard, which is pivotally hinged along its back edge to vary the height of its front edge. An extension plate, or lip, extends outward from the deck's front edge to span the gap between the rear of the trailer bed and the front edge of the deck. Extending from the deck's front edge, the lip rests upon the truck bed to form a bridge between the deck and the bed. This allows personnel and material handling equipment, such as a forklift, to readily move on and off the vehicle during loading and unloading operations.
To help hold the vehicle sufficiently close to the dock platform so that the lip of the dock leveler can remain resting upon and supported by the bed of the vehicle, loading docks often include a vehicle restraint that helps prevent the vehicle from accidentally pulling away from the dock. Vehicle restraints, such as those disclosed in U.S. Pat. Nos. 4,443,150 and 4,915,568, usually include a hook or barrier that restrains the vehicle by reaching up in front of the vehicle's RIG (rear impact guard), also known as an ICC bar. An ICC bar is a beam that extends horizontally across the rear of a truck, just below the truck bed. An ICC bar's primary purpose is to prevent an automobile from under-riding the truck in a rear-end collision.
When a forklift drives over the dock leveler and onto the trailer bed, the weight of the forklift and the cargo it may be carrying can add a significant load to the truck bed. Likewise, when the forklift exits the truck bed, substantial weight is removed from the trailer. Thus, the load carried by the trailer changes repeatedly during the loading/unloading process. The trailer's suspension may respond to these load changes by allowing the trailer and its ICC bar to rise and fall accordingly. The vertical movement can be particularly pronounced when the vehicle has an air suspension system.
As the vehicle moves up and down, the vehicle restraint's hook preferably moves with it to prevent the ICC bar from rising up and over the hook, and thereby disengaging from the barrier. Many barrier restraints can follow the vertical movement of an ICC bar because the barriers are usually mounted to a carriage or sliding member that can travel along a vertical guide track. Unfortunately, vehicles with air suspension often have a generally equal but horizontal component of movement for every vertical movement. Such horizontal movement can apply a substantial horizontal force between the carriage and the guide track. Repeated vertical and thus horizontal movement of the barrier creates localized wear due to the concentrated horizontal line contact between the guide track and the individual rollers of the carriage. Sliding members without rollers distribute the wear more evenly over a broader contact area; however, friction associated with sliding members is typically greater than that of rolling elements, thus wear is a problem with sliding members as well.
Although the vehicle restraint shown in U.S. Pat. No. 4,443,150 has six rollers to help distribute the load, such a design has its drawbacks. If all six rollers are not perfectly parallel to the guide track due to manufacturing tolerances of the size and location of the rollers or nonlinearity of the track, some rollers may carry substantially more load than others. Even if all six rollers are perfectly aligned parallel to a perfectly straight track, an ICC bar pulling the hook forward or pushing the hook down might apply a rotational moment on the carriage such that the upper most rollers push forward on one side of the track while the lowermost rollers press against the rear surface of the track, thereby possibly leaving the rollers of intermediate height only lightly loaded or substantially unloaded. Moreover, if each pair of horizontally displaced rollers is mounted to its own shaft, the added number of shafts can add bulk, weight and cost to a vehicle restraint.
Consequently, there is a need for a vehicle restraint that minimizes the wear between the carriage of a vertically movable barrier and the track along which the carriage travels.